The present invention generally relates to recording/reproduced signal switching systems for 4-head type recording and reproducing apparatuses, and more particularly to a recording/reproduced signal switching system which switches a recording signal and successively supplies the recording signal to four rotary heads upon recording, and which switches signals which are reproduced from a magnetic tape by the rotary heads to produce a single continuous reproduced signal upon reproduction.
Generally, an existing helical scan type recording and reproducing apparatus (VTR) records a video signal by rotary heads onto tracks which are formed obliquely to the longitudinal direction of a magnetic tape upon recording, and reproduces the recorded signal from the tape upon reproduction. Among this type of a helical scan type VTR, there is a known 4-head type VTR in which four rotary video heads are equally spaced apart and mounted on a rotary body such as a rotary drum and a rotary plate, so that adjacent rotary video heads are spaced apart by 90.degree.. In this 4-head type VTR, two mutually opposing rotary video heads have gaps of the same azimuth angle. In addition, the adjacent rotary video heads have gaps of mutually different azimuth angles. Certain conditions must be satisfied in order to form a tape pattern on the tape by the 4-head type VTR, so that the tape pattern is identical to a tape pattern which is formed on the tape by the existing VTR (hereinafter referred to as a 2-head type VTR) which carries out the recording and reproduction by use of two rotary video heads which are mounted diametrically on a rotary body. One condition which must be satisfied, is to select the azimuth angles of the gaps in the four rotary video heads to the same azimuth angles as the gaps in the two rotary video heads of the 2-head type VTR. Another condition to be satisfied, is to wrap the tape around the peripheral surface of the rotary body over an angular range of approximately 270.degree.. Still another condition which must be satisfied, is to select the tape traveling speed so that the tape travels by one track pitch during one field period in which one rotary video head rotates by 270.degree..
If the above conditions are satisfied, the length of the tape which is in contact with the peripheral surface of the rotary body in the 4-head type VTR, becomes equal to the length of the tape which is in contact with the peripheral surface of the rotary body in the 2-head type VTR. In addition, the relative linear speed between the tape and the rotary video head in the 4-head type VTR becomes equal to the relative linear speed in the 2-head type VTR. As a result, one field of the video signal is successively recorded on one video track by the four rotary video heads, and it becomes possible to form a tape pattern on the tape which is completely the same as the tape pattern formed by the 2-head type VTR. Therefore, a compatible tape pattern is formed so that perfect compatibility can be ensured between the 2-head type VTR and the 4-head type VTR.
In the existing 2-head type VTR, the tape is wrapped around the peripheral surface of the rotary body over an angular range which is slightly larger than 180.degree., and the signal is recorded onto or reproduced from the traveling tape by the two rotary video heads. Hence, while one video head is scanning over the tape, the other video head is not in contact with the tape. For this reason, no problems will be introduced even if the recording current is continuously applied to both the video heads. Further, when carrying out the so-called interchanged reproduction in which a tape recorded by one VTR is reproduced by a different VTR, a part of the reproduced signal may become dropped out or a part of the reproduced signal may overlap, due to a deviation in the switching point of the two video heads between the two 2-head type VTRs. The dropout of a part of the reproduced signal or overlap of a part of the reproduced signal, may be prevented from occurring by wrapping the tape over an angular range which is larger than 180.degree. by an angle in the order of 5, and by constantly applying the recording current to both the video heads. By taking these measures, the signal which is recorded immediately before one video head completes recording one track, is simultaneously recorded on a starting part of a subsequent track by the other video head. This measure is the so-called overlap recording.
On the other hand, in the case of the 4-head type VTR, three video heads including the video head which is actually carrying out the recording or reproduction, simultaneously make contact with the tape. Thus, a part of a scanning locus of one video head, overlaps with a scanning locus of another video head. Accordingly, the recording current cannot be constantly applied to the four video heads, as in the case of the 2-head type VTR. In the 4-head VTR, the recording current must be successively switched every time the rotary body undergoes a 270.degree.-rotation, that is, every one field, so as to successively apply the recording current to only one of the video heads at one time.
Therefore, during the recording, switching must be successively performed so as to successively supply the video signal to only one of the four video heads at one time. Similarly, during the reproduction, switching must be successively performed so as to successively switch reproduced signal from the four video heads.
Conventionally, the above switching of the recording signal which is supplied to one of the video heads, and the switching of the reproduced signals from the video heads, were performed by use of relay switches. However, in the 4-head type VTR, the switching must be performed at a high speed which is in the order of one field period intervals and the like. Hence, the use of relay switches in the 4-head type VTR is inconvenient in that chattering is introduced, and the durability of the relay switches became a problem.
The switching of the recording signal which is supplied to one of the video heads, and the switching of the reproduced signals from the video heads, were performed in accordance with a head switching signal. In the existing 2-head type VTR, the head switching signal is a square wave signal. This square wave signal has a duty cycle of 50% and a repetition frequency of 30 Hz, for example, and is obtained based on a pulse which is produced every time the rotary body which is mounted with the two video heads undergoes one revolution. If the scanning period of the first video head corresponds to a low-level period of this square wave signal and the scanning period of the second video head corresponds to a high-level period of this square wave signal, the rotational phases of the heads and the rotary body can be determined. Hence, during the reproducing mode of the 2-head type VTR, it is possible to alternately switch the reproduced signals from the two video heads according to a rotation detection pulse which is obtained by detecting the rotational phase of the rotary body.
On the other hand, in the 4-head type VTR, the rotary body rotates at a rotational speed of 45 rps, for example. However, the four video heads only scan over the tape during a period corresponding to approximately 270.degree.-rotation of the rotary body (approximately 3/4 rotational period), and the heads are not in contact with the tape during a period corresponding to approximately 90.degree.-rotation of the rotary body (approximately 1/4 rotational period). Accordingly, the phase of the video heads and the rotational phase of the rotary body do not coincide. Therefore, there was a disadvantage in that the rotation detection pulse which is obtained by detecting the rotational phase of the rotary body, could not be used to successively switch the reproduced signals from the four video heads.
Furthermore, if the switching is performed in the 4-head type VTR so that the recording current is successively applied to one of the four video heads for every one field period, there was a problem in that the the signal which is reproduced from a part which is in the vicinity of the switching point of the heads may become dropped out during the interchanged reproduction described before.